Passivation method

ABSTRACT

The invention refers to a method for the passivation of zinc, cadmium or their alloys, in particular of zinc-nickel alloys by means of a chrome(VI) free solution containing a weak complexing agent, preferably di- or tri-carbonic acids, preferably chrome(III)-oxalate complex and Co 2+ , whereby the concentration of Co 2+  is greater than 30 g/l.

[0001] The invention refers to a method to passivate zinc and zinc alloy layers as well as cadmium and cadmium alloy layers and claims the priority of German patent application 100 55 215.3, which is referred to for purpose of disclosure.

[0002] It is known to galvanically coat metallic surfaces with metals or metal alloys in order to improve corrosion resistance. Moreover, it is known to further increase the anticorrosive effect of this layer both by its modification and by additional coating systems. A system with good anticorrosive effect is represented by a galvanically applied zinc-nickel alloy, which Is subsequently chromatized and which can additionally be treated with an organic or inorganic coat. A good corrosion resistance is achieved by employing chrome(VI) for passivating the zinc-nickel layer.

[0003] The toxicity of chrome(VI) compounds contrasts their good anticorrosive qualities. Accordingly, it has been attempted for a longer period time to establish chrome(VI) free systems providing a sufficient corrosion resistance.

[0004] For this goal it is known to use chrome(III)-containing passivation solutions as described in the U.S. Pat. No. 4,171,231, which however—due the oxidants also present in the solution—do not lead to chrome(VI) free passivation layers, since chrome(III) is oxidized during the treatment.

[0005] The DE 41 35 524 C2, which is Incorporated into the present description and the teaching of which is fully referred to discloses a passivation method, which achieves Improved anti-corrosion values by a chrome(III)-containing passivation solution with an oxalate complexation (Tab. II; Tab, III). Different examples of chrome(III) compounds for passivation are given In table IV of the mentioned document. The good anticorrosive effect of the passivation method described in this document is based on the use of oxalate as a complexing agent, which—in contrast to other complexing agents—supports the incorporation of chrome into the passivation layer.

[0006] With this type of passivation method one can already achieve good anticorrosion results. Moreover, it Is known to Increase the anticorrosive protection provided by chrome(III)-based passivation methods by the use of cobalt.

[0007] The improvement of anticorrosive protection by the use of cobalt is described in the WO 97/40208, which is hereby incorporated.

[0008] As it can also be seen from this document, the experts aim to produce a conversion layer as compact as possible in order to achieve the layers desired qualities with respect to resistance to corrosion and chemicals.

[0009] The invention has the object to provide both a passivation method with further improved anticorrosive protection characteristics and a respective coating system.

[0010] This aim is achieved by a method and a passivation solution according to the independent claims, Favorable aspects are subject of the dependent claims.

[0011] The invention is based on the finding, that turning away from the efforts of focussing on a compact conversion layer and thus aiming to produce a porous conversion layer leads to improved final products when the porosity is used for binding to at least one further layer.

[0012] The invention can be realized by using cobalt in a passivation solution containing chrome(III) and a weaker complexing agent—preferably a di- or tri-carbonic acid like oxalic acid.

[0013] By using a cobalt concentration greater than 30 μl, preferably a concentration between 70 g/l and 100 g/l, In particular about 90 g/l, (concentrate—12%) already surprising corrosion protection values of 240 h until onset of primary corrosion according to DIN 50961 in the salt spray test according to DIN 50021 SS are achieved in production. Favorably the chrome-cobalt-ratio is 1,7:2,0=0,85.

[0014] The method Is preferably carried out at temperatures up to 55° C. and pH levels of 0,5 to 5,5. Preferably the passivation is carried out at pH 4. Particularly good results can be achieved with a sulfate free solution. This is based on the idea that the use of sulfate leads to a disturbance of the catalytic reaction and interference with the development of an anticorrosive chrome layer.

[0015] The passivated layer is re-treated with an organic or inorganic coating which penetrates into the porous conversion layer. Preferably, a coat of Aquares™ (Enthone-OMI GmbH, Neuss) is applied to the passivation layer. A particularly good corrosion control can be achieved by a double Aquares layer. The Aquares layer surprisingly is no longer traceable in the REM in some variants of the invention.

[0016] Both the characteristics of corrosion control and resistance to chemicals can be further improved by means of a Topcoat containing a dry lubricant. Also this coat according to the invention engages in a special binding to the porous conversion layer. Hereby, the desired friction and flow values can achieved at the same time.

[0017] Surprising properties are displayed by a combination of the dry lubricant Molykote® D-7100 (Dow Corning) and the zinc-nickel alloy passivated according to the invention. In comparison to known coating systems this system is particularly resistant to chemicals and rim cleaner. It is thus especially suitable for wheel screws. Furthermore a surprising temperature insensitivity of the combination according to the invention has been stated.

[0018] Preferably this dry lubricant layer is applied onto a double Aquares layer. 

1. Method for the passivation of zinc, cadmium or their alloys, in particular of zinc-nickel alloys by means of a chrome(VI) free solution containing a weak complexing agent preferably di- or tri-carbonic acids, preferably chrome(III)-oxalate complex and Co²⁺, characterized in that the Co²⁺ concentration is greater than 30 g/l.
 2. Method according to claim 1, characterized in that the Co²⁺ concentration is 70 to 110 g/l.
 3. Method according to claim 1, characterized in that the Co²⁺ concentration is about 90 g/l.
 4. Method according to one of the preceding claims, characterized in that the passivation is carried out at 20° C. to 55° C.
 5. Method according to one of the preceding claims, characterized in that the passivation is carried out at pH 0,5 to 5,5.
 6. Method according to one of the preceding claims, characterized in that the passivation is carried out at pH
 4. 7. Method according to one of the preceding claims, characterized in that the ratio of chrome to cobalt is 1,7: 2,0.
 8. Method according to one of the preceding claims, characterized in that the solution is sulfate free.
 9. Method according to one of the preceding claims, characterized in that the passivated layer is treated by an additional coating.
 10. Method according to one of the preceding claims characterized by a coating with Aquares™.
 11. Method according to one of the preceding claims, characterized by a double coating with Aquares™.
 12. Method according to one of the preceding claims, characterized in that the passivated layer is treated with a dry lubricant.
 13. Method according to one of the preceding claims, characterized in that the passivated layer is coated with an organic Topcoat.
 14. Method according to one of the preceding claims, characterized in that the passivated layer is treated with a PTFE-containing coat.
 15. Method according to the preceding claim, characterized in that the passivated layer is coated with Molykote® D
 7100. 